The present invention relates generally to mechanisms for indicating the state of a valve used in oil and a gas production operations and, more particularly, to mechanisms for indicating the position of a reciprocating member in a reciprocating-type valve used in undersea wellheads and Christmas trees. Still more particularly, the present invention relates to a reliable apparatus for indicating the position of the reciprocating member in an undersea, hydraulically actuated, fail-safe gate valve.
Gate valves used in undersea wellheads and Christmas trees and underwater templates/manifolds for offshore oil and gas production operations often are provided with hydraulic actuators, controlled from a hydraulic power supply on a platform, to operate the valve gate. Such hydraulic actuators are frequently of the fail-safe type, in which pressurized hydraulic fluid is supplied to the actuator from the power supply through control lines or the like in order to force a piston downwardly within the actuator, and the gate along with it, thereby aligning a port in the gate with flow ports in the valve, opening the valve. If the pressure on the hydraulic fluid is removed, either intentionally or accidently, for example, through failure of the hydraulic power supply, return springs lift the piston and gate upwardly, thereby aligning a solid portion of the gate with the flow ports of the valve, closing the valve.
Valve position indicators are useful in day-to-day operations for verifying proper functioning of the hydraulic actuator. Situations sometimes arise wherein, for any of several reasons, the valve or the actuator jams, leaving the valve open when it should be closed or closed when it should be open or somewhere therebetween. Often it is necessary to know the position of the gate in order to diagnose and correct the problem. Accordingly, a position indicator for a gate valve performs a necessary and important service.
Prior art position indicators for subsea valves tend to rely upon a detailed mechanical and/or electrical structure. See, e.g., U.S. Pat. No. 3,522,596 (Fowler et al). Fowler et al disclose the use of read switches in connection with magnets to indicate valve position on a remote position read out device on a surface platform. The reed switches or the magnets are attached to a component which is moved in response to upward and downward motion of a valve stem. When the valve is fully open or fully closed, a corresponding reed switch is in proximity to a magnet and an electrical signal is communicated to the platform, actuating an indicator lamp or the like to inform the operator of the present valve position. (FIGS. 1-3 and 6-8). This type of apparatus can also be used to transmit information as to the rotational attitude of a rotating shaft. (FIGS. 9 and 10). Cams and microswitches, or other suitable information relay means, may be used in place of the reed switches. (FIG. 5).
In general, the more structurally detailed a device is, the more susceptible it is to operational failure. The rigors of installation and operation in an undersea oilfield environment tends rapidly to take its toll on delicate mechanical instrumentation. Electrical instrumentation, also, is susceptible to failure in the hostile undersea environment. Equipment failures must be diagnosed and the failed apparatus must be repaired or replaced, all of which is very time consuming and expensive in undersea oilfield operations. Thus, an ideal valve position indicator is simple and reliable.